Is Earth's Temperature Up or Down or Both?

Is Earth's Temperature Up or Down or Both?

Scientists Investigate Reasons for Temperature Trend "Disagreements"
Between Layers of the Atmosphere

February 6, 1997

Thermometers on the ground, measuring the near-surface
air temperature, demonstrate a marked increase in globally-averaged temperature
over the past two decades. Computer models of global warming predict that
the temperature trend in the Earth's thick lower atmosphere, called the
lower troposphere, should be experiencing an even more pronounced
warming that increases smoothly with altitude. And yet, satellite observations
of the temperature of the Earth's lower troposphere do not reveal any overall
warming trend. Although interpreted by some as a controversy, research from
NASA/Marshall Space Flight Center
and the Global Hydrology and Climate
Center now suggests that the temperature structure of the atmosphere
is more complex than we (and our computer models) originally thought.

These results will be presented today (February 6) at the 77th meeting
of the American Metorological Society in Long Beach, California in a special
session dedicated to the scientific study of global warming.

Dr. Roy
Spencer, a scientist at NASA/Marshall and principal author on the paper,
has been monitoring the temperature of layers in the Earth's atmosphere
from space. Along with Dr. John Christy
of the University of Alabama in Huntsville,
Spencer has produced a temperature record spanning 18 years. Acquired from
Microwave Sounding Unit (MSU) instruments flying aboard the TIROS series
of weather satellites. Their data show temperature variations in the lower
troposphere, a region from the surface to about 5 miles into the atmosphere.

"The temperatures we
measure from space are actually on a very slight downward trend
since 1979 in the lower troposphere. We see major excursions due to volcanic
eruptions like Pinatubo, and ocean current phenomena like El Nino, but overall
the trend is about 0.05 degrees Celsius per decade cooling," Spencer
remarked.

However, temperature measurements on land and ocean
are up. "Thermometers taking the temperature at the surface show a
warming trend of about +0.10 to +0.15 degrees Celsius per decade,"
Spencer continued. "Current computer models of global warming always
predict that the temperature variations at the surface should increase smoothly
with height as you go up through the lowest 8 miles of the atmosphere."
This should make the temperature trend in the troposphere not only upward,
but more pronounced than on the surface.

But the space-based measurements show
a more complex vertical structure, with cooling in the lower portion
of this deep layer and warming in the upper portion. Spencer and co-author
Dr. William Braswell of Nichols Research
Corporation have great confidence in the quality of their satellite
data. "We've concluded there isn't a problem with the measurements,"
Spencer explained. "In fact, balloon measurements of the temperature
in the same regions of the atmosphere we measure from space are in excellent
agreement with the satellite results."

"Instead, we believe the problem resides in
the computer models and in our past assumptions that the atmosphere is so
well behaved. These models just don't handle processes like clouds, water
vapor, and precipitation systems well enough to accurately predict how strong
global warming will be, or how it will manifest itself at different heights
in the atmosphere," remarked Spencer.

These poorly modeled processes are all related to
convection. This is the continual overturning of the atmosphere that occurs
as water, evaporated from the Earth's surface, carries excess heat energy
into the upper atmosphere where it can be more efficiently radiated to outer
space. This convective redistribution, the scientists theorize, may be part
of what causes the interesting height-dependent structure in the temperature
variations seen in the MSU data. Spencer says that the models also suffer
from "numerical diffusion," wherein water vapor in the lower atmosphere
is allowed to unrealistically diffuse into the upper atmosphere, where it
acts as a greenhouse blanket. "All of these effects together make the
computer-modeled atmosphere look much more vertically uniform than it probably
is," Spencer concluded.